I will often buy ground sirloin out of the managers special section of the super market. Tasty but dry. I have been adding a tablespoon of a olive oil/canola oil blend to moisten it a bit. Looking for better sugestions, taste is the main objective.
Try using a panade.
From Cook’s Illustrated:
I tend to use panko in my panade (as I never have white bread in the house). Again, Cook’s Illustrated:
Bacon fat.
Stop buying ground sirloin. Buy something fattier, easy peasy!
I prefer to buy chuck or brisket and grind it myself in the food processor, but sometimes one, including me, buys on price. The trick is then making something inexpensive better. Sirloin has a great, beefy flavor, but it is lean.
I get it for 1/2 price.
I like this trick from America’s Test Kitchen, assuming you’re making burgers:
Scatter the ground beef on a cookie sheet and put it in the freezer. Take it out when it’s very cold, maybe a little stiff here and there, but not actually frozen. Dribble melted butter all over it. The cold meat will solidify the butter into bits and streaks. Now quickly mold the beef into patties and cook them.
Mistermage has been buying 97% lean ground chuck and it sucks for making burgers (it’s fine for sloppy joe’s). So I use for 2 lbs of the 97%: 2 eggs, 1/2 cup of oatmeal, some diced onions and chopped peppers (non hot kinds) and a couple splashes of soy sauce and/or worchestshire, a teaspoon of garlic and maybe a shake of salt (he has high BP so I tend to cook w/o salt and let everyone add their own).
So our hamburgers on the grill are really “meatloaf” on the grill but they are very tasty and, most importantly, moist!
That sounds like my turkey burger recipe.
Chuck is not that lean, even trimmed. I know one can find very, very lean ground beef, but I’d be really surprised if it came from the chuck. Chuck is actually the best to get the 80/20 mix that makes such great burgers. Quickly googling, the super-lean ground beef is probably eye round. If it’s being labeled ‘ground chuck’, it’s probably mislabeled.
This may work, especially if you use bacon grease instead of butter.
Ooops, you are right… it’s 93% ground beef. My defense is I don’t buy the groceries; I merely cook them. :o
Still probably the eye round, just not trimmed as well. 
I like the lean stuff for tacos, tomato sauces, and a few other select uses. Burgers need the 80/20 stuff. Meatloaf and meatballs need the 85/15, maybe the 90/10, but also need ground pork and ground veal.
Is there not a local butcher’s where you can buy beef fat/trimmings?